1. Field of the Invention
The present invention relates to subsea wellhead and pipe connectors, and more particularly to axially locking connectors for tying back to subsea wellheads with well conductor or riser pipe. Even more specifically, the present invention concerns a passive wellhead tieback connector having an internal lock mechanism which is externally mechanically actuated such as by a remote operated vehicle (ROV) controlled tieback actuator tool. Even further, the present invention is provided with an adjustment mechanism which can be adjusted on the working deck of the drilling and production vessel or spar or adjusted in the subsea environment to develop a high pre-load force of the lock mechanism during connector installation, enabling the releasable connector to withstand loads generated by spars, tension leg platforms (TLP""s) and any other floating riser support structures.
2. Description of the Prior Art
The development of offshore petroleum oil and gas deposits from undersea wells involves drilling production wells in the sea bed from a drilling platform, and then capping the wellhead at the ocean floor until a production platform, either stationary of floating, is put into place on the surface of the ocean. To commence production from a subsea well, large diameter marine riser pipe is run downward from the production platform and connected to the subsea wellhead, a procedure known as tying back to the wellhead.
Several types of tieback connectors are available to connect or tie back production risers to wellheads. Certain of these connectors require rotation of a riser string to lock them to, and release them from, the wellhead housing. However, when rotating to the left to unlock the connector, the joints in the riser string tend to unthread and loosen. Reconnecting these loosened joints can be a serious and costly problem to the operator.
To solve this problem, tieback connectors that are actuated by axial movement have been developed to provide a connection to, and disconnection from a wellhead without rotary motion. In certain of such connectors, a pre-load force can be imposed through the connector""s lock ring and onto the wellhead housing. Prior devices also include adjustment of the pre-load force through cumbersome changes between the relative positions of the inner body and outer body forming such connectors. However, such connectors are not constructed to provide an adequate pre-load force between a lock ring on the connector and the wellhead, and may not be adequate to maintain the locking force when extreme production fluid pressures are encountered which tend to separate the riser from the wellhead.
One approach is disclosed in U.S. Pat. No. 5,259,459 to Valka titled xe2x80x9cSubsea Wellhead Tieback Connector,xe2x80x9d which is directed to a wellhead tieback connector actuated solely by axial motion to achieve connection and disconnection from the subsea wellhead using a type of expanding lockdown ring and a type of adjustment assembly. After the connection is made between the tieback connector and the wellhead, the apparatus taught by this patent is used to effectuate a rigid lockdown, thereby eliminating any slippage that exists in the manufacturing or installation tolerances in the riser pipe being connected.
The advent of spar-type floating production facilities has increased the need for a premium, high force-resistant, tieback connection system for affixing a riser pipe conduit from pre-drilled subsea wellheads to completion trees at the surface within the spar""s structure. One unique problem that a spar presents is the limited space from which to lower and install a riser pipe conduit and tieback connector since the inside diameter of the pipe will only permit passage of equipment 26 inches in diameter or smaller.
In addition to the small profile requirements, the subsea tieback connection system must be resistant to extreme external bending and axial loads in addition to the pressures generated from the well. A tieback connection system is required which can generate sufficient locking force to resist separation forces in excess of 800,000 pounds, which is often referred to as a connector""s pre-load force.
To generate this force in a tieback connector, the present invention provides a structure wherein the relative location between a recessed groove in the wellhead and a lock ring forming part of the tieback connector can be readily adjusted to provide maximum pre-load. The lock ring is actuated to expand into the wellhead groove, and beveled engagement surfaces on the lock ring and wellhead groove interact in cam-like fashion to develop the necessary pre-load force.
The tieback connector of the present invention is considered xe2x80x9cpassivexe2x80x9d in that it does not incorporate an internal hydraulic or otherwise powered mechanism for accomplishing locking and unlocking thereof with respect to a subsea wellhead. In accordance with the present invention, there is provided a tieback connector that has a tubular outer connector body that is adapted to rest axially upon an upper surface of the wellhead. The tieback connector has an inner body that is adapted to extend partially into an inner diameter of the wellhead. A lock ring, being a split ring having spring-like characteristics, extends circumferentially around a portion of the inner body and is adapted for expansion into locking engagement with internal locking geometry of a wellhead component for establishing locking connection of the tieback connector to the wellhead. An energizing mandrel is in linearly moveable assembly with the tieback connector and has an elongate tubular extension that extends axially between the wellhead and the inner body, with a lower end of the tubular extension oriented for expansion of the lock ring. The energizing mandrel is moved linearly for expanding the lock ring into the internal locking geometry of the wellhead and thus lock the tieback connector to the wellhead. An elongate tubular adjustment element or ring extends around and is operatively connected to the inner body, the adjustment ring positioned beneath and in positioning and supporting contact with a lower annular surface of the lock ring. The tubular adjustment element is capable of axial movement relative to the inner tubular body of the tieback connector to alter the axial position of the lock ring relative to the inner body to establish an adjustable tensile pre-load force on the tieback connector as the lock ring is forced into fully engaged locking engagement with the internal locking geometry of the wellhead. One or more tubular elements, including a tubular lock positioning element are subjected to axially compressive force, developed by the cam-like activity of the lock ring with the internal locking geometry of the wellhead, and thus then to yield or buckle to provide a cushioning activity or compressive spring pre-load force.
The structure of the present invention provides a significant mechanical advantage between a linearly moveable lock actuator assembly and the lock ring which compresses the lock ring into the internal wellhead locking groove. Further, the tieback connector of the present invention is specifically constructed whereby mating locking parts under compressive force in the tieback connector bend and/or buckle to create a tensile pre-load force acting on the inner tubular body of the tieback connector.
To accomplish a high force-resistant tieback connection pursuant to the above objectives, the expanding lock ring of the connector is positioned a short distance above the internal recessed locking groove within the wellhead such that upon contact, the tapered shoulders between the lock ring and wellhead groove stretch the inner connector body down until the lock ring fully enters the internal locking groove, thus developing sufficient tensile force to generate a desired pre-load. The relative position of the lock ring to the internal wellhead locking groove is adjusted by a threaded tubular adjustment member or ring having axial positioning and supporting relation with the lock ring. Rotation of the adjustment member on the inner tubular body imparts axial movement to the lock ring to accommodate differences in machining tolerances between the wellhead housing and the tieback connector and to pre-apply the desired amount of tensile pre-load force to the inner tubular body of the tieback connector.
To provide the necessary mechanical advantage between the lock ring and the lock energizing mandrel which expands the lock ring into the wellhead groove, with application of minimal force with the energizing mandrel, which minimal force can be applied by a ROV, a tapered lock ring actuation shoulders are provided on the tubular extension of the energizing mandrel and on the lock ring which are in contact as the energizing mandrel is actuated. When the lock ring action shoulders or surfaces pass by each other during the locking process, a small relative angle is taken by the load path, resulting in a significant mechanical advantage between the two parts, in the range of 27:1 in the preferred embodiment of the invention. By way of example, in one embodiment of the present invention, a linear force applied by a ROV on the externally exposed drive members of the energizing mandrel generates approximately 29,500 pounds of downward force, which translates to 810,000 pounds of pre-load locking force acting on the lock ring.
A further feature of the present invention is to provide certain parts having a design geometry such that these parts bend or buckle to create a compressive spring pre-load force. This compressive spring force is introduced by making the tubular adjustment element or ring and an adjustable tubular lock positioning element long and slender, whereby compressive deflection thereof is provided under load. Since both of these elements are fully captured on all sides by more rigid components, the deflection or buckling of these two parts is restrained against failure and therefore the two parts are fully supported. The stored energy of the adjustable tubular connector element and the tubular lock positioning element, in combination with the stretch associated with axially loading the tieback connector""s main body provide the necessary stretch and stored energy for generating the required pre-load force.
The oil and gas production fields that are being tied back to the surface are getting larger and larger. Currently the tieback connectors are actuated hydraulically, such as is shown in commonly assigned U.S. Pat. No. 5,775,427, covering an internally locked subsea wellhead tieback connector. If the internal hydraulically energized lock actuation mechanism of the tieback connector can be eliminated, thus rendering the tieback connector passive, the cost of manufacturing the tieback connector can be significantly reduced. When a portion of the energizing mandrel of the tieback connector is externally exposed for actuation by a ROV or other similar equipment, the tieback connector can be actuated for locking and unlocking without sacrificing its functional integrity.
So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the preferred embodiment thereof which is illustrated in the appended drawings, which drawings are incorporated as a part hereof.
It is to be noted however, that the appended drawings illustrate only a typical embodiment of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.